The present invention relates to a multitransmitter digital signaling system in mobile communications wherein one service area is divided into a plurality of zones, a carrier wave of the same frequency is assigned to all base stations, and angularly or phase modulated by common digital information signals and simultaneously transmitted at respective base stations.
When digital information signals are transmitted to a service area from a plurality of base stations it is preferable that the base stations simultaneously transmit the digital information signals at the same carrier frequency because efficient frequency utilization may be attained and channel switching when a mobile station moves from one zone to another may be eliminated. However this system has a disadvantage that fading occurs in an overlapped zone due to the interference among carrier waves transmitted from different base stations so that degradation of reliability in transmission results.
In order to overcome this problem there has been proposed a system wherein the base station transmitters are synchronized so that the frequencies of the carrier waves may be completely synchronized. However this system is complicated and very expensive. There has also been proposed a system wherein the center frequencies of the carrier waves which are within the same band are offset by a suitable amount as much as, for instance, the signal transmission rate. In this system the frequency fluctuation of carrier waves must be minimized in order to maintain the frequency difference among the carrier waves within a prescribed range. Furthermore when more than four zones are overlapped, the stability requirements of the carrier wave frequency become very severe. Moreover, because of the difference in center frequency of the carrier waves, reliability of signal transmission varies from zone to zone.
There has also been proposed a system wherein the modulation indices or deviation ratios in the adjacent base stations are offset by a suitable amount. However this system still requires high stability of carrier waves when more than four zones are overlapped. Furthermore because maximum frequency deviation is different in each zone, reliability of signal transmission is different from one station to another.